FIG. 1 is a highly simplified view of a digitizing tablet 3 and its associated stylus 6. There are four current-to-voltage amplifiers (labeled I-to-V), one at each corner. The stylus 6, when activated, produces a signal 9, as indicated in FIG. 2. This signal induces currents I1-I4, which are detected by the I-to-V amplifiers.
The I-to-V amplifiers each produce a voltage indicative of the size its respective current. Processing circuitry, known in the art, receives the voltage signals, and computes the position of the stylus signal 6.
The currents I1-I4 are induced because the stylus 6 acts as one plate of a capacitor. The digitizing tablet (which bears a resistive surface of a material such as indium tin oxide) acts as the other plate. As FIG. 3 indicates, when negative charge is applied to the tip 2 of the stylus 6, a positive charge is induced on the surface of the tablet 6. Currents I1-I4 supply this positive charge.
Conversely, as in FIG. 4, a positive charge on the tip 2 of the stylus 6 induces a negative charge on the tablet. The currents I1-I4 supply this negative charge.
Each current can be viewed as following a direct path from the stylus to one corner of the tablet. The resistance of each path depends on the length of the path. The size of the current traversing each path depends on the resistance. Thus, knowledge of the current allows one to infer the path length. A highly simplified example will illustrate.
Assume that the stylus in FIG. 5 represents a voltage source applied to the tablet. Four currents will be generated, each indicated by a dashed arrow running to a corner. The paths have the relative lengths shown in FIG. 5, and also listed the following Table.
TABLE ______________________________________ RELATIVE RELATIVE RELATIVE RELATIVE I-to-V PATH LENGTH RESISTANCE CURRENT VOLTAGE ______________________________________ AA 0.66 0.66 1.52 1.52 BB 1.00 1.00 1.00 1.00 CC 0.93 0.93 1.08 1.08 DD 0.55 0.55 1.82 1.82 ______________________________________
The resistance of the path depends on the length, so that the relative resistances are in proportion to the relative path lengths, as indicated in the Table.
Current equals voltage divided by resistance, V/R, so that the relative currents will be inversely proportional to the relative path lengths, as indicated in the Table.
The I-to-V amplifiers in FIG. 1 each produce a voltage which is proportional to its respective current. Thus, the outputs of the I-to-V amplifiers have the relative magnitudes indicated in the Table. The output signals can be used to compute position of the stylus, as stated above.
Noise
The I-to-V amplifiers produce other, unwanted, signals, in addition to those generated by the stylus. For example, nearby cathode ray tubes, fluorescent lights, and electric motors induce charge on the digitizing pad. Many of these unwanted signals cause the I-to-V signals to each increase by a fixed value, at any given time. That is, these unwanted signals add a common-mode component to the I-to-V signals.
The common-mode signal interferes with computation of stylus position.